The helion, the nucleus of a helium-3 atom, consists of two protons but only one neutron, in contrast with two neutrons in common helium. Its hypothetical existence was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while he was working at the University of Cambridge Cavendish Laboratory. Oliphant had performed experiments in which fastdeuterons collided with deuteron targets (incidentally, the first demonstration of nuclear fusion).
Helium-3 was hypothesized to be a radioactive isotope until helions were also found in samples of natural helium, which is mostly helium-4, taken both from the terrestrial atmosphere and from natural gas wells. This was done by Luis W. Alvarez andRobert Cornog in cyclotron experiments at the Lawrence Berkeley National Laboratory in California in 1939.
Although helium-3 was found to be about 10,000 times rarer than helium-4 in the helium from the gas wells, its significant presence in underground gas deposits implied that either it did not decay, or else it had a very long half-life – billions of years.Hydrogen-1 and helium-3 are the only stable nuclides that contain more protons than neutrons.
Helium-3 occurs as a primordial nuclide, escaping from the Earth's crust into the atmosphere and into outer space over millions of years. Helium-3 is also thought to be a naturalnucleogenic and cosmogenic nuclide, one produced when lithium is bombarded by natural neutrons. Those are released by spontaneous fission and by nuclear reactions withcosmic rays. Some of the helium-3 found in the terrestrial atmosphere is also a relic of atmospheric and underwater nuclear weapons testing, conducted by the three big nuclear powers before 1963. Most of this comes from the decay of tritium (hydrogen-3), which decays into helium-3 with a half life of 12.3 years. Furthermore, some nuclear reactors(landbound or shipbound) periodically release some helium-3 and tritium into the atmosphere. The nuclear reactor disaster at Chernobyl released a huge amount of radioactive tritium into the atmosphere, and smaller problems cause smaller releases. Furthermore, significant amounts of tritium and helium-3 have been deliberately produced in national arsenal nuclear reactors by the irradiation of lithium-6. The tritium is used to "boost" nuclear weapons, and some of this inevitably escapes during its production, transportation, and storage. Hence, helium-3 enters the atmosphere both through its direct release and through the radioactive decay of tritium. The vast majority of these two gases have been produced and leaked by the former Soviet Union, Russia, the United Kingdom, and France.
Helium-3 is proposed as a second-generation fuel for nuclear fusion in hypothetical fusion power plants, but such plants are still very early in their development—especially since first generation reactors have not yet entered into service. Helium-3 can be used in instruments for the detection of free neutrons, such as neutrons leaking from nuclear reactors.
Because of its lower atomic mass of 3.02 atomic mass units, helium-3 has some physical properties different from those of helium-4, with a mass of 4.00 atomic mass units. Because of the weak, induced dipole–dipole interaction between helium atoms, their macroscopic physical properties are mainly determined by their zero-point energy (ground-state kinetic energy). Also, the microscopic properties of helium-3 cause it to have a higher zero-point energy than helium-4. This implies that helium-3 can overcome dipole–dipole interactions with less thermal energy than helium-4 can.
The quantum mechanical effects on helium-3 and helium-4 are significantly different because with two protons, two neutrons, and two electrons, helium-4 has an overall spin of zero, making it a boson, but with one fewer neutron, helium-3 has an overall spin of one half, making it a fermion.
Helium-3 boils at 3.19 K compared with helium-4 at 4.23 K, and its critical point is also lower at 3.35 K, compared with helium-4 at 5.2 K. Helium-3 has less than one-half of the density when it is at its boiling point: 59 gram per liter compared to the 125 gram per liter of helium-4 — at a pressure of one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026 kilojoule per mole compared with the 0.0829 kilojoule per mole of helium-4.